Really Bad Gas Mileage.

Hey Guys I need some help.My buddy has a 98 Spyder Non Turbo. And he is getting terrible gas mileage. What do you guys think it could be?
To give you an idea how bad....

If he fills his tank up he can drive about 40 miles before it drops down to 3/4 and just under 100 miles driven the car is already half empty.

The car is completely stock. No mods whatsoever. It has new spark plugs and a new air filter. The new filter seemed to have helped alittle he said. The car has 40,000 miles on it.

Let me know anything you can think of to correct the problem.
Oh and the last thing he doesnt really drive the car hard. He does his share of speeding around town but he doesnt drive nearly as hard as I do in my 99 GST and I get way better gas mileage then he does. WE just cant figure it out.

Thanks for the help guys.

his 02 sensor(s) could be going bad i know since mine has went bad and i havent' replaced it i get really shitty gas mileage... just a thought...

Gauge could be off. Tell him to count the miles and then remember how many gallons he fills up and then divide them and then he will know what his MPG truly is.
That is the only way to really find out if it is bad or not, IMO

R u sure he aint slammin the gas pedal every time he hits a turn lol. i use to do that on my fist car.

does he have a CEL light? with milage that bad it should be on. my guess would be a bad o2 sensor. or you just need to reset the ECU.

The stock fuel gauge isn't linear. You might get 300 miles to a tank, but 100 miles will drop it to 1/2. Calculate actual milage, not "half way down on the gauge"

hit the tripmeter and run it till almost e on the gauge and see how many miles your actually getting... my fuel gague isnt working right now (need to trim my white gauges) so i just fill up and hit the trip and fill back up when it hits 200 to be sure...

edit- my 96 non turbo usually got more than 300 miles per tank if i remember right.. i know i averaged it a few times and averaged like 28-31.. i live in the country so its not all stop and go but i drive into the city almost everyday and im in no way light on the gas.. iv never noticed my gas milage get totally slaughtered by peggin it now and then..